The present invention relates to a process for processing electric steelworks dusts and zinc-bearing, high-iron, oxidized materials in a low-frequency induction furnace of a coreless type, resulting in:
reduction of contained iron oxides to cast iron, PA0 concentration of non-ferrous metal oxides in fumes recovered from the process, and, PA0 formation of slag suitable for disposal in normal dumps. PA0 recovering, in metal form iron contained in EAF dust; PA0 separating and concentrating, without losses, Zn and Pb oxides and other either metal or non-metal impurities (Cd, F, Cl, and so forth); PA0 removing all other components of dusts, sending them to form a slag suitable for being disposed of to normal dumps, i.e., with no toxic or noxious character. PA0 a reductive step, in which the active agent is coal contained in cast iron, kept rapidly moving by the inductive phenomenon (steering). The interested zone is the bath/pellets contact region, which is the hottest region in the furnace and a large amount of energy must be supplied; PA0 an oxidative step, which takes place inside the dusts, in which blended coal burns producing CO and generating the necessary energy to keep the high temperature value and to allow iron oxides to oxidize Zn vapors rising from bottom.
The cast iron bath is kept at a high temperature and is rapidly circulated by the induced currents which flow through it. This bath constitutes the "reducing section" of the furnace. The reduction of the oxides of metal which constitute the feed: Zn, Pb, Cd, Fe, takes place inside this bath.
The bath covering layer, occupied by the feedstock (pelletized steelworks dusts, granular coal, slagging agents), constitutes the oxidizing section of the furnace inside which coal is burnt yielding CO, and vapors of Zn and other feedstock metals leaving the bath are oxidized again and turn into fumes.
A proper fume collection unit collects the dusts and recovers Zn, Pb, Cd oxides concentrated to more than twice as high levels relatively to their initial concentrations at process beginning.
It is well-known that iron scrap smelting in electrical furnace causes 10-20 kg of dusts to be formed for each ton of produced steel. These light-weight, fine and easily dispersible dusts (referred to hereinafter as "EAF dusts") contain, as their oxides, 20-25% Fe, 18-25% Zn, 2-4% Pb, and still other, more or less dangerous impurities.
On considering the size of present steelworks, the amount of dusts produced is extremely large and, owing to health, environmental and economic reasons, a process for neutralizing these dusts and for recovering valuable metals therefrom is urgently required.
It has already been attempted to recycle the dusts to the same furnace which produced them. This allows iron to be recovered and oxides of nonferrous metals to be gradually concentrated in the fumes. But greater difficulties were soon experienced in the steelworks. In furnace charging and fumes collection, the energy consumptions increased, and the throughput of furnaces decreased. As a result, the method is regarded as not being desirable for economic and, above all, environmental, reasons.
At present, for processing EAF dusts use is made of several thermal processes in a reducing environment. These processes are carried out in rotary furnaces, hearth furnaces, plasma furnaces and flame furnaces.
The most widely known and used process is the Waelz-Berzelius process in which the dusts are mixed with fine coal, lime and silica and the resulting mixture is fed to a long rotary furnace.
The reduction takes place in that portion of the furnace in which temperatures of 1200.degree. C. are reached: Zn and Pb turn into vapors and are recovered as dusts in condensers and in fumes filtrations units, whereas iron oxide, by reacting with the slagging agents, produces a slag which can not always be disposed of as an inert material.
Zn and Pb in Waelz oxides reach concentration levels of 60% and 10%, respectively. Therefore, this material is suitable for recovering both metals.
The HTR process, conceptually similar to the preceding one, achieves a certain energy saving by feeding the dust to that portion of the furnace in which temperatures of 1400.degree. C. are reached. FeO is not reduced and turns into slag.
The plasmadust process (used in Sweden) uses a nontransferred arc plasma, causing a gas to flow through an electric arc discharge that is maintained between two electrodes installed inside the tuyeres of a metallurgical furnace which is top fed with coke flowing from top downwards. Pelletized dusts with coal and flux are injected into the flame of plasma torch. Iron oxides are reduced to cast iron, and Zn and Pb are reduced and volatilized, and are collected in an outer condenser. Lime and silica react with the other dust components, turning them into slag. At present, this process is mainly used for processing the dusts from electrical furnaces used to produce stainless steel.
The Inmetco process (used in the USA) is preferably used for processing the dusts from stainless steel producing electrical furnaces. Use is made of a rotary table furnace inside which the pelletized mixture of dust with coal and fine coke is submitted to a first reduction. Zn and Pb concentrate in the fumes from this furnace. The pre-reduced pellets, which contain all iron, are charged, together with scrap and rolling scales to a submersed arc electrical furnace to obtain an iron alloy with Cr. Ni and Mo contained in the original dusts.
A St. Joe reactor is a vertical steel furnace with a water jacket subdivided into two steps. The burner, is fed with coke powder and with oxygen enriched air, so a particularly high-temperature flame is generated (approximately at 2000.degree. C.).
The metallurgical charge is pneumatically injected to the reducing region of the flame. Zn, Pb and Cd are reduced, vaporized, and collected in a sock filter.
The high-iron slag is granulated and can be sold to cement factories (in order to add iron to cement mixes), or is used as a raw material for blast furnaces.
The Tetronics process by British Steel Co. uses a transferred-arc plasma furnace. The plasma torch is installed on the furnace crown and, in order to distribute energy to the bath, it can rotate with a variable inclination relative to the vertical axis.
The fumes from stainless steel, mixed with 28% of anthracite, are continuously fed to the furnace (with a feed rate of 500 kg/h) at a constant temperature of about 2000.degree. C. Cr, Ni and Mo are recovered as iron alloys.
The exhaust gases are deprived of dusts inside sock filters.
Also EAF dusts containing 18% of Zn were tested, with ZnO concentration up to 60% being obtained.
Also the Kaldo process for Boliden, developed for lead bearing residues, and the Slag Fuming process, very well-known for Zn and Pb recovery from silica slag from wind furnaces, can be regarded as representing possible solutions for EAF dusts processing.
In the vertical electrothermic furnace developed by St. Joe Minerals, studied for zinc minerals, EAF dusts may be charged, and sinter then mixed with the residue from blendes roasting.
The resulting sintered product flows then through a pre-heating rotary furnace and then enters the electrothermic furnace from the top. The coke feeds the furnace with energy and creates a conductive region for electrical energy supplied by means of graphite electrodes. Under the operating conditions of the furnace, Zn volatilizes. The fumes are sent to bubble through a cooled Zn bath which acts as a condenser. The feed should not contain less than 40% of Zn. The amount of EAF dusts which can be processed in this process is limited by the large amounts of impurities they contribute.
Finally, Michigan Technological University tried to add pellets of oxidized residues together with cast iron and scrap to a cupola furnace. By operating at 1510-1538.degree. C., a slag is formed and iron is reduced and recovered in the liquid state, while Zn and Pb are volatilized and recovered as raw zinc oxide. It should be considered that pelletized oxides only represent 5% of the charge fed to the cupola furnace.
The facilities for performing the cited process require high investment costs and the resulting financial burdens are such that they can be supported only by primary steelworks or consortia.
The produced metals must be reprocessed in order to exploit them at a commercial level.
Only some from the above processes produce slags suitable for being disposed of in normal dumps.